Normal TSH Levels
A normal Thyroid-Stimulating Hormone (TSH) level ranges from 0.4 to 4.0 mIU/L, as recommended by the American Medical Association and American College of Physicians. 1
Understanding TSH Reference Ranges
TSH is the primary screening test for thyroid dysfunction, with different ranges indicating various thyroid conditions:
- Normal TSH: 0.4-4.0 mIU/L
- Subclinical Hypothyroidism: Elevated TSH with normal Free T4
- Overt Hypothyroidism: Elevated TSH with low Free T4
- Subclinical Hyperthyroidism: Low TSH with normal Free T4
- Overt Hyperthyroidism: Low TSH with elevated Free T4 1
Important Considerations for TSH Interpretation
Physiological Variations
- TSH levels can vary by up to 50% on a day-to-day basis
- Variations occur based on age, race/ethnicity, and sex
- In older adults (70-79 years), the reference range may be slightly broader (0.4-5.9 mIU/L) 1, 2
Diagnostic Approach
- A single abnormal TSH value is insufficient for diagnosis
- Serial TSH measurements are essential to confirm thyroid dysfunction
- Free T4 should be measured as a follow-up test when TSH is abnormal 1
Clinical Significance of TSH Values
Low TSH Values
- Undetectable TSH (<0.04 mIU/L) is associated with thyrotoxicosis in 97% of cases (excluding patients on thyroid hormone therapy)
- TSH values between 0.04-0.15 mIU/L may not always indicate hyperthyroidism; 41% of patients with these values show no signs of hyperthyroidism 3
High TSH Values
- TSH >10 mIU/L generally warrants treatment for subclinical hypothyroidism
- TSH between 4.5-10 mIU/L requires more individualized assessment 1
Common Pitfalls in TSH Interpretation
Relying on a single measurement: TSH should be repeated to confirm abnormalities due to its natural variability 1
Ignoring age-related changes: Slightly higher TSH values may be normal in older adults 2
Laboratory interference: Heterophilic antibodies may cause falsely elevated TSH values 2
Non-thyroidal illness: Severe illness can affect TSH levels without indicating thyroid dysfunction 4
Medication effects: Glucocorticoids and other medications can suppress TSH without causing hyperthyroidism 4
When to Consider Additional Testing
- When TSH is abnormal, Free T4 should be measured to distinguish between subclinical and overt thyroid dysfunction
- Anti-thyroid peroxidase (anti-TPO) antibodies may be considered to identify autoimmune etiology, though routine measurement is not strongly recommended 1
While some researchers have proposed lowering the upper TSH reference limit to 2.0-2.5 mIU/L, current evidence does not support this change. Classifying individuals with TSH between 2-4 mIU/L as abnormal may lead to unnecessary treatment and potential harm 5.